If you're seeing a prompt of that nature, suggest contacting YT. Various file converters are available on the Net, but you shouldn't need them.
Something is amiss. There's no indication that YT charges for mp4 or any other format. Mp4 has been uploaded successfully here without charge.
An article states: "According to Youtube, the best video format is MP4 with H. 264 video codec and AAC audio codec. You get a high-quality video and a small file size."